GPS altitude readings going wild

I have the exact same problem. I rode 80 kilometers yesterday and it measured 19,800 meters of elevation gain. I did what the article recomended says to no avail.

The same problem, the altimeter goes crazy  7xss

Cleaned, washed, didn't help

Update on this. This is not a calibration issue (which I did try with no expectation of success, and indeed it didn't work). It's completely bonkers altitude readings. I contacted Garmin Customer Support. They suggested to check for debris and rinse with warm water. This didn't fix it (same as others experienced). They said if that doesn't work they can offer me a replacement for £218 (since it was out of warranty). So I'm assuming they know that failure of the pressure sensor is a problem. I took the replacement option since otherwise I have a watch that will always give me a crazy Training Stress Score and that I can't sell. I see that the Fenix 8 has a guard over the pressure sensor holes, which could be a response to a frequent problem.

Sean C said:

I am at a loss as to why the watch would NOT use GPS data for altitude? 

Because the vertical accuracy of GPS is miserable (unlike the horizontal accuracy). If you know something about trigonometry, and see how low the orbits of GPS satellites are, you probably understand the reason.

Garmin tells the following about the GPS altitude accuracy, in the article Elevation Accuracy of Outdoor and Fitness Devices | Garmin Customer Support

Elevation calibrated by GPS is accurate to +/-400 feet with a strong GPS signal.

trux Thank you.  Another assumption about GNSS debunked.  I'm familiar with the math, but didn't realize there were accuracy differences between 2D and altitude.  Now I do, and this explains so much about ALL of the GPS receivers I've used.

This from another vendor ~2021

"Vertical Accuracy

Most receiver manufacturers do not provide a vertical (altitude/elevation or z-value) accuracy specification. Just as a general observation, the vertical accuracy of GNSS/GPS receivers is typically 1.7 times the horizontal accuracy. For example, a receiver with 1 m 2DRMS horizontal accuracy would likely provide closer to 2 m vertical accuracy. Again, this estimate is based on general observation of several different receivers, not thorough testing of any one receiver. "

It isn't just absolute altitude accuracy that matters (2 meters wouldn't be very good for that but acceptable), but precision of elevation changes as well. The latter is used for things like calculating number of floors climbed or calculating vertical gain during activities, or vertical speed. GPS based altitude is just too "noisy" for that to give you a meaningful data. In other words, GPS based elevation data is accurate but not precise. In contrast, barometer based elevation data is not accurate without calibration but it is very precise for measuring elevation changes. 

Yes, that's correct, thought the range of inaccuracy that Garmin specifies, is not 2 meters, but rather "+/-400 feet with a strong GPS signal", it means +/-122 m. And it can go wild when the signal of some satellites drops out.

Hello, I have the same issue with same data - minus 8000 meters to 5000 meters. I have Fenix 7s solar edition. It has started on 15th April.